20ft span what size lumber

[Don P]

 It's easier to get a solid connection in steel for that heeljoint and steel is good in tension.
What is the roof pitch going to be?

[Don P]

Here's one way of looking at it. The peak of the truss needs to support around 3000 lbs. This is a pic of the forces at a 4/12 pitch, probably the lowest pitch you would use, add 2 zeros to all the numbers, load is the 3000 lbs in black hanging from the peak, which becomes 4700 lbs compression heading down each leg of the toggle and produces 4400 lbs of tension in the bottom chord and at the heeljoint connections, kingpost is carrying zero load;



If simple is the main goal and given that you can weld, that bottom tension chord could be a piece of pipe or rod welded to angles at each end. One angle clips leg down down over the post plate beams. Another angle is back to back with that, leg up and pockets the end of a pair of rafters. The pipe is welded to the angles across the building, a rigid triangle. The ridge beam is secured well between the rafter tops.  There would be a rafter on each side of the pipe and a filler block between the rafter pair to make it a rigid assembly, the canted post heading up to support the ridge beam. This is similar to what we're getting ready to do today on the barn repair. I'm going to weld a turnbuckle in the middle of the bottom chord so we can pull that puppy back together while we're at it. Not high art but it takes care of the connections well. A 20' one inch pipe is going to sag in the middle, there's where the kingpost dangles down and holds it back up.
 

[1953belair]

The pitch will probably be alittlem than what's on drawing you posted.....so a pipe would help alot? I also have some 3"inch angle iron to.at first I thought about doing angle and sandwiching it between two wood beams and hiding it would this be a better route? 

[1953belair]

I'm going to put as much pitch on the roof as the 14ft rafters will let me with about a 1ft of overhang past the side beams maybe a 8/12 

[Don P]

That would work fine and would be easier to hide. With a steeper pitch the tension in the bottom chord and at the connections decreases. Notice how much the forces drop at 8/12.





 The trick with most trusses is keeping that low end of the rafters, the heeljoint, from drifting outward under the load from above. It is hard to simply pin or bolt the bottom and top chords together adequately to resist that thrust in wood. That is the reason for that upturned angle in my last pic or for burying the top chord into a large notch like in my timberframe pics above. This just got a lot better at that pitch though, a 3/4" bolt through a SYPine 3 piece joint, double rafters sandwiching a single bottom chord is good for about 1500 lbs so a 2 bolt connection is starting to work without steel, but the steel heel is a more rigid connection. Just stuff to think about as you design it. I'm off to load and hit the steel shop, I'll check on you tonite.

[1953belair]

Well I doubt that I will be able to get that much pitch im just going to get theses beams up after I figure out how I want to build them then mut my 4x6 in the middle of the beam and get the most I can out of the 2x6x14 rafter

[1953belair]

Would a small cable also work In between 2x10 if so what is the best method to connect it and get it nice and tight..the angle iron I have the most of is almost 1/4" think and 3".it is pretty heavy I can get different stuff tho my uncle works at a steel plant

[1953belair]

Also I do plan on doing a birds mounts notch on the rafters and maybe some collar ties to keep rafters from trying to spread apart.on my porch roof I did collar ties on every other rafter but it's also alot heavyer roof with plywood and shingles but it's not as wide either.....sorry for all the post just been thinking about it in between work breaks 

[Don P]

Deducting 1-1/2" from the 1' overhang and another 1-1/2" for half the doubled ridge I'm at 12'11-1/16" at 8/12, so that should work with 14' material.

The level seat cut on your birdsmouth should be no more than 2-1/2" to stay within the max 1/4 rafter depth notching rule.

They've modernized roof terms, or defined them more clearly in the last 20 years or so. A collar tie is in the upper third of the roof height and its job is to keep the ridge from blowing apart in high wind. We are supposed to either put at least a 1x4 across every other rafter pair in that zone or run a metal strap up and over the ridge nailed to the opposing rafters to hold the rafters and ridge together in high wind. If you remember the films of the roofs flying apart during Andrew it is to restrain that. A collar tie is too high to restrain the spreading forces at the rafter feet.

A rafter tie is a ceiling joist or tie across the rafters in the lower third of roof height. It's job is to restrain the thrust we have been talking about. If those are not on every rafter pair then a ridge beam is required. That's what you are doing between your 3 trusses.

Just as an aside, doesn't matter here but for others reading along, in a conventional roof as a rafter tie is raised off the plate to that 1/3 height limit, the rafters need to get deeper and the heeljoint connection needs to get better, that is all spelled out in the footnotes in the span tables in the rafter chapter of the building codebook.

Securing and tensioning a cable would be a job. I'm having a hard time right now visualizing hiding steel in this, long day :

Drawing it out and then trying to lay out bolts, there isn't sufficient room once you allow for sufficient edge and end distances and spacing between. Nails would work, a .131x3" nail in SYP is good for 106 lbs in single shear in 1-1/2" material, that is a common size gun nail, its the diameter of an 8d common hand drive but in 3" length. It would take about 12 from each rafter into the tie if it is assembled like this;

 


I drew that without fill between the rafters, I would fill it to make it stiffer and to keep the bees out of there.

Supporting the ridge beams is another detail to think about, one way could be hangers nailed to the trusses and drop in 10' sections of beam between trusses.

[1953belair]

With that pitch will 2x10s on the outside of the 4x6 middle post carriage bolted to the 2x10s work if I box it in with 2x4s top and bottom inside/in-between the 2x10s or can I put 10ft 2x10s together with  plywood to make up the Gap and put them in between the 2x10s on each side of the 4x6 post so that it makes it a solid beam.....that way I can leave a hole in the middle for the 4x6 beam to go In and would make a good connection with the carriage bolt.

[Don P]

 Basically like so?
I drew the bottom chord as 2x8 it can be 2x10 if you like, the key is to get the number of nails in that heeljoint connection. put ply or 1/2" material between the rafters to get them to the 4x6 thickness and pinch the top of the 4x6 as in the pic. The bottom connection of the kingpost is not that critical here so carriage bolts would work, they are really not considered a structural bolt because they are threaded all the way, but as we talked about the bottom chord is really a tension element here and the king isn't really doing much work at the bottom chord. You can pack in whatever fill between the bottom chords. In that situation I would probably stop the king short of the bottom about 1" and slip in a flat 2x4 hanging it down about 1/2" rather than trying to make it flush make it a stepped trim detail.

[1953belair]

That's pretty much what I was thinking I have got 2x10s yesterday evening...the bottom cord the 2x10s I haven't decided if I want to sister them together with plywood or do them like your drawing on each side of the 4x6...I like that because the 4x6 has a pocket to set in-between but I feel like the 2 2x10s butted together with plywood wood make a stronger beam and would resist warping better, just don't have the connection to the king 4x6 in the middle of the bottom cord,if I did it that way I would probably make a metal T bracket to bolt the bottom cord to the king post better,witch do you think is better

[Don P]

The critical connections are first the bottom chord to rafter heeljoint, then the top chord rafters to kingpost, then the ridgebeam support and finally the bottom of the kingpost to bottom chord.

The kingpost top the way it is drawn requires some fussing but it is then pretty easy to nail a concealed flange double joist hanger to it for the ridgebeams to drop into. All just stuff to roll around in your head.

[1953belair]

Yes I no it lots of different ways to do it....thanks for all the help I'm going to work on it this weekend hopefully il get alot done

[Don P]

Good luck, supposed to be even hotter over the weekend. When I built my shop I figured you should be able to hang a truck underneath a truss, but nobody would loan me their truck .

[1953belair]

 

 

 

 

 

[1953belair]

Haven't got as much done as I would like to but I thought I'd post a picture of the progress...the 4x6 in middle of beams is notched for the double 2x8 ridge beam to sit inside of it and on the other end the bottom it slides in-between the 2x10s the thickness of the osb I put in between the two beams.... should work out good 

[Southside]

What kind of wood is that in the roof structure?

[1953belair]

It is all pine I just been staining it to give it a better look

[Don P]

Looking good. Don't forget to brace well in the other direction from the posts to bottom chords as well.

[1953belair]

Yea I've got them cut out already....I'd like to do a arch on one side of the braces they will be 4x6 post .might be to much Time cutting them out though.going to try to cut one on a scrap piece and see how it goes

[Hilltop366]

Nice work, it always seems to take longer than you think it well.

I would be tempted to add X bracing in between the trusses along the centre line as well.(from king post to king post).

[Weekend_Sawyer]

Don, It's a pleasure to watch you cipher!

Mr Belair, that's going to be a nice looking structure.

Jon